In the diagnosis and treatment of various diseases as well as in preventative health checkups, it is becoming increasingly important to monitor the concentrations of certain ions (e.g. cations) in a patient's body. One cation which has merited considerable attention is potassium. High serum potassium levels are known to cause changes in muscle irritability, respiration and myocardial functions. Low potassium levels can cause excitatory changes in muscle irritability and myocardial function. Therefore, serum potassium determination has become an important diagnostic tool in the diagnosis and treatment of illness.
A useful tool in assaying for potassium has been the ion-selective electrode. One type of ion-selective electrode useful in determining ion concentration in body fluids has an electrode body (usually a glass or plastic container) containing a reference solution in contact with a half-cell of known potential (a reference electrode) and an ion-selective membrane located in an aperture in the electrode body. The ion-selective membrane is mounted in such a fashion that, when the electrode is immersed in the unknown solution, the membrane contacts both the reference and unknown solutions. A metal probe coated with a layer of insoluble salt of the metal in the reference solution and immersed therein serves as one of the contacts for measuring the potential between the electrodes and provides a reference potential for the electrode. The sensitivity of the electrode to an ion in solution is determined by the composition of the membrane. This type of electrode is referred to in the art as a "barrel" electrode.
A significant advance in the ion-selective electrode art is the dry-operative electrode described in U.S. Pat. No. 4,214,968 (issued July 29, 1980 to Battaglia et al). Prior to the discovery of such dry-operative ion-selective electrodes, electrodes had to be either stored in an aqueous solution or treated with aqueous solution just prior to use in an ion-activity-determining operation. The term "dry-operative" refers to an ion-selective electrode which provides reproducible potentiometric determination of ion activity, which is related to the ion concentration of an aqueous test solution, with no requirements for wet storage or preconditioning prior to use.
One of the specific ion-selective electrodes disclosed in the examples of Battaglia et al is a potassium ion-selective electrode. This ion-selective electrode comprises a hydrophobic ion-selective membrane coated over a reference electrode. The reference electrode consists of an electrolyte layer and a metal/metal salt reference electrode. The metal/metal salt reference electrode comprises a conductive layer of a metal in conducting contact with a layer of salt of a metal and a dry electrolyte layer in contact with the metal salt layer.
The second member of the metal/metal salt reference electrode comprises the electrolyte layer. Preferably, the electrolyte layer is a dried hydrophilic layer.
The dried electrolyte solution of the ion-selective electrode comprises a hydrophilic binder having a salt in solid solution therewith. Preferably, the anion of the salt is common to the salt of the metal salt layer and the cation of said salt comprises the ion which the electrode is designed to detect. Hence, potassium chloride is the salt described and used solely in the examples of the Battaglia et al patent (as well as other disclosures pertaining to potassium ion-selective electrodes).
It has long been accepted that for optimum performance in detecting potassium ions with a potassium ion-selective electrode which uses silver chloride as the insoluble metal salt, potassium chloride is the best choice as the electrolyte since it is known that the electrode responds best to potassium ions. Therefore, although mixtures with other salts may be used, potassium chloride is a logical choice in a potassium selective electrode which uses silver chloride as the insoluble metal salt.
A substantial amount of salt other than the potassium salt would not be expected to provide equivalent precision and accuracy due to decreased available potassium ions at the reference layer/membrane layer interface.
However, a problem encountered with the use of this prior art electrode is that the adhesion between the ion-selective membrane and the metal salt layer is poor. This causes some electrodes to delaminate. Electrodes in this condition cannot be sold. This leads to wasted materials because these ion-selective electrodes must be discarded.
There is, therefore, a need for a dry-operative potassium ion-selective electrode which maintains excellent precision and accuracy and provides good adhesion between the coated layers.